Science News has a nice article on how diamonds are formed and surface. A Gemstone’s Wild Ride:
Almost all diamonds come from volcanic deposits called kimberlites. Tens of thousands of such deposits, some more than a kilometer across, have been found protruding through Earth’s surface. However, only one in every 200 of them contains gem-quality diamonds. Increasingly, scientists are looking at kimberlites’ odd mix of geological characteristics to discern how diamond deposits get there in the first place.